strange shadows after removing doubles
You have Smooth Shading enabled. Select the object and switch to Flat Shading and it will look like before (Object > Shade Flat).

The other software split the connection between faces, similar to how Blender's Edge Split modifier works, to make the faces appear flat shaded. This is because the vertex normals are depending on the face normals of the neighboring faces. If the faces and edges are split from each other this doesn't happen, because there is only one face the vertex is part of. If you now interpolate the normals for the shading of the face, it will remain flat shaded even when you use Gouraud/Smooth shading. 

A better approach that doesn't require the separation of faces and therefore duplication of vertices, is to simply not interpolate the face normals over the barycentric coordinates. That is what Blender's Shade Flat does.